In the fabrication of semiconductor components, the various devices are formed in layers upon an underlying substrate that is typically composed of silicon, germanium, or gallium arsenide. The discrete devices are interconnected by metal conductor lines to form the desired integrated circuits. The metal conductor lines are further insulated from the next interconnection level by thin films of insulating material deposited by, for example, CVD (Chemical Vapor Deposition) of oxide or application of SOG (Spin On Glass) layers followed by fellow processes. In such microcircuit wiring processes, it is essential that both the insulating and metal layers have smooth topographies, since it is difficult to lithographically image and pattern layers applied to rough surfaces.
Chemical/mechanical polishing (CMP) has been developed for providing a smooth semiconductor topography. CMP can be used for planarizing: (a) the substrate; (b) insulator surfaces, such as silicon oxide or silicon nitride, deposited by chemical vapor deposition; (c) insulating layers, such as glasses deposited by spin-on and refIow deposition means, over semiconductor devices; or (d) metallic conductor interconnection wiring layers.
Briefly, CMP involves holding and rotating a thin, reasonably flat, semiconductor wafer against a rotating polishing surface. The polishing surface is wetted by a chemical slurry, under controlled chemical, pressure, and temperature conditions. The chemical slurry contains a polishing agent, which is an abrasive material, and chemicals chosen to selectively etch or oxidize particular surfaces of the wafer during CMP. It may be desirable to perform the chemical etching and mechanical abrasion steps simultaneously or sequentially. The combination of chemical etching and mechanical removal of material during polishing results in superior planarization of the polished surface.
One problem area associated with chemical/mechanical polishing is in the area of slurry preparation and storage. As the polishing slurry is a suspension of a mechanical abrasive in a liquid chemical agent, the suspension is normally premixed as a batch and stored in a batch tank of about 350 gallon capacity. The premix tank is connected to another tank, commonly called a "day tank," of about 50 gallon capacity by an extensive plumbing system. The plumbing system allows the slurry to be continuously re-circulated between the day tank and the premix tank. This approach hopes to minimize settling and agglomeration of the particulates in suspension and maintain a reasonably uniform mixture. In total, a single batch of slurry may have a volume of about 350 gallons. Once mixed, the slurry "brew" needs from about one-half hour to 12 hours of vigorous agitation/recirculation before the slurry is useable in order to insure reasonable uniformity.
The effectiveness of the CMP process depends significantly upon the physical parameters, for example: chemical concentration, temperature, pH, specific gravity, etc., of the slurry. Because of the large slurry volume, i.e., 350 gallons, it is extremely difficult to make small adjustments to the mixture in a timely manner. Additionally, the large volume of slurry commits the CMP equipment to a particular process, e.g., planarization of a metal layer, because of the incompatibility of the chemicals in a particular slurry mix to the planarization of a different material, e.g., a dielectric. Modification of the chemical properties of the total slurry mix is very difficult. Therefore, a large quantity of highly refined and expensive chemicals must be ultimately used, stored or disposed. Environmentally, this presents a significant challenge as the CMP apparatus is changed from material process to material process, e.g., polishing substrate, to polishing metal, to polishing dielectric, etc. Of course, the changeover process is time consuming and ultimately very expensive when the high cost of the potentially wasted slurry and lost processing time is considered.
Accordingly, what is needed in the art is a slurry preparation apparatus that: provides a continuous slurry delivery system, constantly monitors the slurry parameters, and continually adjusts the slurry components to correct the slurry parameters to nominal values.